This project will describe quantitative and qualitative aspects of the organization of projections from the dorsal cap (DC) and ventrolateral outgrowth (VLO) of the principal olivary nucleus to both the cerebellar flocculus and nodulus and the vestibular nuclei in albino rabbits. This will yield a precise picture of the efferent organization of the DC-VLO and clarify structural features defined by these projections in the vestibulocerebellum and vestibular nuclei. A study with multiple retrograde labeling methods is designed to determine the relative numbers and distribution of DC-VLO cells that project (1) to the flocculus and either the medial or superior vestibular nucleus, (2) to the nodulus and either the medial or superior vestibular nucleus, (3) to the flocculus and nodulus and (4) to either one or all of the structures. A second set of experiments is designed to identify the course and pattern of termination of DC-VLO axons in the vestibulocerebellum and vestibular nuclei. Both anterograde degeneration and anterograde autoradiographic tracing methods will be employed. The tracing of anterograde degeneration after local chemical lesions of DC-VLO with 3-acetyl pyridine will demonstrate the course and morphology of axon terminals in the vestibular nuclei and cerebellar cortex; autoradiographic experiments using either [3H]-D-asparate or a cocktail of tritiated proline, leucine and fucose will provide corroborative evidence and eliminate the possibility of a fibers-of-passage problem from the injection micropipette in the degeneration experiments. The data will refine the description of structural features of the flocculus, nodulus and vestibular nuclei that are defined by DC-VLO projections. These experiments are also motivated by a need to determine if olivo-bestibular connections can be transected without damaging the main climbing fiber projections to the vestibulocerebellum. If surgical interruption is possible, this system can be used for a model for investigations of the physiological role of olivary collateral projections to deep cerebellar nuclei.